Hi,
Tried to use kvmclock with xenner and noticed that the kvmclock
(MSR_KVM_SYSTEM_TIME msr) is incompatible with xen.
kvm guests do this to translate the tsc delta into nsecs:
#define get_clock(cpu, field) per_cpu(hv_clock, cpu).field
static inline u64 kvm_get_delta(u64 last_tsc)
{
int cpu = smp_processor_id();
u64 delta = native_read_tsc() - last_tsc;
return (delta * get_clock(cpu, tsc_to_system_mul)) >> 22;
}
whereas xen guests do this (64bit version):
static inline u64 scale_delta(u64 delta, u32 mul_frac, int shift)
{
u64 product;
if (shift < 0)
delta >>= -shift;
else
delta <<= shift;
__asm__ (
"mul %%rdx ; shrd $32,%%rdx,%%rax"
: "=a" (product) : "0" (delta), "d" ((u64)mul_frac));
return product;
}
Note that xen does a 64bit multiply (of the 64bit delta and the 32bit
factor) yielding a 128bit result, then picking bits 32-95 for the 64bit
return value. In contrast kvm does a simple 64bit multiply, which is
equivalent to using the lowest 64 bits. Thus kvm is off by factor 2^32,
and that without even considering the ordering of two (shift + multiply)
operations and any rounding errors ...
cheers,
Gerd
--
http://kraxel.fedorapeople.org/xenner/
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